1. Field of the Invention
The present invention relates to a semiconductor light emitting element such as a light emitting diode, a semiconductor laser, etc. and a fabrication method thereof.
2. Description of the Related Art
To improve the luminance of a semiconductor light emitting element such as a light emitting diode, a semiconductor laser, etc., it is extremely important to efficiently extract the light emitted by the active layer of the light emitting element to the outside of the element. That is, it is necessary to restrict light reflection on the surface of the light emitting element as much as possible to let out the emitted light to the outside of the element and increase the so-called light extraction efficiency.
As a means for restricting light reflection on the surface of the light emitting element and increasing the light extraction efficiency, there is a method for restricting total reflection on the surface of the light emitting element. The ratio between the light emitted through the element surface and the light total-reflected on the element surface is determined by the refraction index of the surface layer of the element and the refraction index of the outside world (including a transparent protection layer or the like). As the difference in the refraction index between the surface layer and the outside world is smaller, the critical angle is larger. The critical angle is an angle of light incidence to the interface between the surface layer and the outside world. Assume that the refraction index of the surface layer is n11 and the refraction index of the outside world is n12. In this case, the critical angle θ is expressed by the following mathematical expression 1.=sin−1(n11/n12)  (Mathematical Expression 1)
As obvious from the mathematical expression 1, as the difference between the refraction index n11 of the surface layer and the refraction index n12 of the outside world is smaller, that is, as the ratio n11/n12 is closer to 1, the critical angle θ takes a larger value (a value closer to 90°). Light having a larger angle of incidence than the critical angle θ is total-reflected on the interface and thus is not let out to the outside. Accordingly, as the difference between the refraction indexes is smaller, the ratio of the light to be total-reflected is smaller, so that more light is emitted to the outside world resulting in achieving a higher light extraction efficiency.
However, a general light emitting element is formed of a surface layer made of gallium arsenide or the like having a refraction index of 2 to 4, which is molded with resin having a refraction index of about 1.5. Since the difference in the refraction index between the surface layer and the outside world is relatively large, the light extraction efficiency is relatively low. Therefore, various methods for improving the light extraction efficiency have been developed.
As one of such methods, there is a technique for forming a light scattering layer having recesses and bosses on its surface, on the optical window from which light is extracted, as disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H10-163525 and Unexamined Japanese Patent Application KOKAI Publication No. H11-46005. By forming the light scattering layer having recesses and bosses on its surface, it is expected that total-reflection of the light on the surface of the light scattering layer be restricted and the light be efficiently emitted to the outside of the element. Furthermore, there is a technique for forming a thick window layer on the upper surface of the light emitting layer, as disclosed in U.S. Pat. No. 5,233,204.
However, the formation of such a scattering layer and window layer has problems in the viewpoints of workability, repeatability, etc. For example, according to the light scattering layer forming method disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H10-163525, scattering particles need to be uniformly dispersed. According to the light scattering layer forming method disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H11-46005, air bubbles need to be uniformly dispersed in a liquid-like film. However, it is extremely difficult to carry out the dispersion with a good repeatability and fabricate light emitting elements having a high uniformity and a desired luminance at a high yield. Moreover, if the window layer disclosed in U.S. Pat. No. 5,233,204 is grown thick by an epitaxial method, the crystal might be deteriorated.